In a mobile communications network a geographical area is covered by a number of cells. For example, in a Long Term Evolution (LTE) network a number of evolved UMTS terrestrial radio access network (eUTRAN) cell sites cover a geographic area. The eUTRAN cell sites are positioned such that continuous LTE coverage is achieved in said geographic area.
FIG. 1 shows an example of eUTRAN cell sites, showing a group of neighboring cells 101 to 107, with one of the cells 101 showing a division of the cell into 120-degree sectors. FIG. 1 is conceptual, and in reality the radio coverage does not form true hexagonal coverage areas. Cell sizes differ per area (for example in urban areas compared to rural areas), and cells will partially overlap to prevent white spots. In heterogeneous networks, micro cells are embedded within the coverage of macro cells. Frequency allocation to the respective sectors of the cells in a geographic area is arranged such that there is no or minimal interference by antennas using the same frequency. Further details about this type of communication will be familiar to a person skilled in the art, and defined by the third generation partnership project (3GPP) technical specifications relating to the 36.xxx series, which provide further information about eUTRAN radio transmission specifications.
Terminals in use by subscribers residing in one particular cell, and having established a functional radio connection with the appropriate sector of the eNodeB of that cell, continuously measure the signal strength of said appropriate sector of their current cell, as well as the signal strength of sectors of adjacent cell(s). When a subscriber is moving then, at some moment in time, handover (when engaged in a communication session) or location update (when not engaged in a communication session) will take place from the current cell to an adjacent cell.
Referring to FIG. 2, when a user equipment device is moving from cell 101 towards cell 103, the signal strength of cell 101, as detected by the user equipment device of the subscriber, decreases, whilst the signal strength of cell 103, as detected by that same user equipment device of the subscriber, increases. When the signal strengths are equal, a handover or location update is initiated for this user equipment device, to hand the user equipment device over from cell 101 to cell 103.
The “size of a cell” is determined by, among others, the transmission power as provided by the transceiver to the sector antenna and the signal amplification by the sector antenna. A cell will thus have a maximum radius (or “distance” in the case of non-spherical cell size) within which it can serve user equipment devices. The size of a cell is implicitly also determined by the signal from neighboring cells. When considering two adjacent cells, as depicted in FIG. 2, the boundary of the respective cells is formed by their intersection (whereby it shall be understood that the adjacent cells will partially overlap; their “intersection” will hence be formed by an area rather than by a line). A user equipment device residing within the coverage of cell 103 could be served by cell 101. However, since the user equipment device detects a stronger signal from cell 103 than from cell 101, it will camp on cell 103.
Frequency planning and transmission power level planning are typically static. They are determined based on the geographical characteristics of the area, expected cell use, including the number of simultaneous calls, data traffic etc., per cell or per sector. Frequency planning is also a carefully executed activity.
Static planning of frequency allocation and transmission power cannot take ad hoc usage patterns into account. For example, when the number of subscribers residing in a cell (and being engaged in communication activity) reaches a particular threshold, there will be congestion in that cell (or the chance of congestion in that cell). As a consequence the cell will not be able to continue serving all subscribers residing in the cell for voice/video calls or for data services.
Techniques exist whereby neighboring cells can take over the service of a particular cell. Consider, for example, neighboring cells 101 and 103 of FIG. 2. When the base transceiver station in cell 101 is (temporarily) non-operational, neighboring cell 103 can increase its transmission power. The increase of transmission power by cell 103 would in such case only be applied for the sector facing cell 101. Subscribers residing in the coverage area of cell 101 can then automatically start camping on cell 101 and 103. When cell 101 has become operational again, cell 103 can revert to its original transmission power level. Other neighboring cells for cell 101 would behave in similar fashion as cell 103, namely temporarily increasing their transmission power for the sector facing cell 101.
While the above-described method is devised for the case that cell 101 has become non-operational, it involves changes in transmission power level. This may have a far-reaching impact on cell planning as a whole. The adjustment of a transmission power level for one or more eNode-Bs may, in addition, affect Automatic Neighbor Relation (ANR) tables in the involved eNode-Bs. Hence, it is generally not desirable to adapt the transmission power level or frequency allocation for short-term, usage-driven adaptation to the network.
Another method of mitigating the problematic effects of high traffic in a particular cell is to revert to a lower quality voice codec or video codec, as appropriate. Adapting the codec will, however, be a complex process, as the subscribers in this particular cell who are engaged in a voice/video call, have negotiated a voice codec with a remote party. Voice codec is not under control of the eNode-B, but is instead controlled by the application, such as voice over LTE (VoLTE). Likewise, reducing data bearer throughput, for data sessions other than voice/video calls, cannot be undertaken autonomously by the eNode-B. Besides that, reverting to a lower quality codec or reducing data bearer throughput has a direct impact on end-user service level.
From the above it can be seen that existing techniques for dealing with congestion in a cell of a mobile communications network can lead to other problems, such as adversely affecting frequency re-use techniques in situations where power levels are changed, or service level quality in situations where codec quality is changed.